Compare Measurements: 0.3 cm vs 3 mm Length Conversion

To solve this problem, we'll focus on converting the measurements to the same unit:

• Step 1: Convert $0.3$ cm to millimeters. Since $1$ cm \equals $10$ mm, we have:
• $0.3 \, \text{cm} = 0.3 \times 10 \, \text{mm} = 3 \, \text{mm}$.
• Step 2: Compare the converted measurement ($3$ mm) with the given $3$ mm.

We find that both measurements are equal.

Therefore, $0.3$ cm is exactly equal to $3$ mm.

Conclusion: Hence, the correct choice is $=$, indicating both values are equal.